Small form factor disk drives are growing in commercial importance as they assume the functions of larger, costlier, high performance direct access storage devices (DASD). As drive size continues to decrease, they are also becoming practical for a number of other applications requiring information storage. A small form factor disk drive generally includes one or more disks, each having a pair of recording surfaces for storing information. Information is accessed and retrieved from each recording surface with a transducer. The data capacity of the drive is determined by the number of recording surfaces available for user data, and the amount of data that can be stored on each data recording surface.
Two predominant and competing objectives in the development of disk drive technology have been to maximize data capacity, while minimizing drive dimensions for reduced space requirements. These interests must be balanced in new drive designs, since a reduction in size has a direct impact on data capacity. Ultimately, a particular application will determine which factor is to be given the highest priority.
A number of small form factor drives are currently available with disk diameters typically ranging from 51/4" to 1.3" in diameter. Many of these are available in enclosures meeting industry standard form factors and functional interface requirements to easily accommodate a variety of different applications. For example, IBM Corporation offers a 1.6" high 31/2" form factor disk drive having two stacked disks, magnetoresistive heads enabling high areal densities, and a data capacity of 8.7-10.8 gigabits. Versions are available with industry standard SCSI (small computer system interface) and SSA (serial storage architecture). The product is suitable for applications requiring high data capacities, e.g. multimedia systems or array subsystems emulating larger high performance DASD. At the other end of the spectrum, Hewlett Packard's 1.3", 20-40MB, single-platter Kittyhawk drive targets applications with small storage requirements, such as the palmtop, fax machine, cellular phone and printer markets.
A variety of applications exist wherein drive height is a primary concert and data capacity is secondary. For example, an emerging industry standard is the credit card-sized PCMCIA type form factor established by the Personal Computer Memory Card Industry Association. The standard was introduced to encourage development of thin, multi-application memory cartridges compatible with PCMCIA-defined computer slots. It has therefore become an objective of a number of competing disk drive manufacturers to provide low profile disk drives meeting these form factor requirements. Three PCMCIA form factor types have presently been defined. Type III card measures 10.5 mm in height, 85.6 mm in length and 54 mm in width. The dimensions of a type II card are approximately 5 mm high.times.86 mm long.times.54 mm wide. A type I card is a modest 2.5 mm h.times.85.6 mm 1.times.54 mm w.
Disk drives utilizing 1.8" disks or smaller may meet the length and width requirements of a PCMCIA type III card, but only one commercially available disk drive to date have met the rigid height limitations of the type II card, and none have been implemented for type I. Drive height is generally determined by the height of the motor and the actuator assemblies, and these heights, in turn, are determined by the number of data surfaces in the device to be accessed. Consequently, drive height can be minimized by providing a single disk with only one or two data surfaces. For applications requiring small disks (e.g. 1.8" or smaller), the data capacity attainable for a single sided disk had been impractical. However, recent developments in MR head technology have now made such designs feasible.
A single-platter, single-sided disk drive of any dimension offers a number of benefits. Already mentioned is the low profile height attained by limiting the actuator to a single suspension. Another benefit is that of center parking. Disk drives occasionally experience external shock forces during movement or operation, as discussed, for example, in an article entitled "Improved Low Power Modes Highlight 1.8-in. Drives". Richard Nass, Electronic Design, Apr. 18, 1994, pp. 47-54, p. 48. External shock may cause the sliders to slam into the data surfaces, resulting in damage to the disk or and the delicate read/write elements, as well as the loss of data. To avoid this risk, most disk drive designs employ a method for "parking" the suspension and head in a safe place during periods of inactivity.
U.S. Pat. No. 5,231,549 discloses a method for loading each head onto a ramp located at the outer diameter of the recording surface. A problem with this approach, however, is that the largest outer diameter data tracks are sacrificed to allow the overhang of the ramp. The design also requires either a slightly larger hard disk enclosure or special positioning of the actuator pivot to accommodate the ramp, making it less suitable for low profile form factors. In addition, the design requires a specially adapted suspension, e.g. a tab or load/unload rod appended to the suspension beam.
An alternative design provides a "landing zone", or region not used for data storage, at the inner diameter (ID) of the disk. For instance, U.S. Pat. Nos. 5,291,355 and 5,313,350 disclose single-platter disk drives equipped with a mechanical latching mechanism on the actuator for securing the heads at the innermost track of the disk. U.S. Pat. No. 5,291,355 describes the use of a magnetic field to capture the actuator arm and park the heads at the desired zone. Other applications provide a textured landing zone to minimize friction between the parked head and disk substrate. See, for example, anonymously submitted research disclosure "Head Parking Zone", RD29563 Nov. 1988 N. 295. Texturing reduces the amount of torque required to "unpark" the head, i.e. to break the slider free of the landing zone.
Parking at the ID is desirable because the data tracks are small, and loss of their use only minimally impacts a disk's data capacity. It would be even more desirable to park the head at the center of the disk, where tracks would be too small for practical use. Most disk drive designs preclude center parking, since a spindle hub occupies the center region. A single-sided disk could be designed with a hub flush with the plane of the data surface and suitable to center-parking. Alternatively, the single platter could be mounted directly to a flat motor, eliminating the need for a hub. In either implementation, a structure may be provided at the disk center or on the drive enclosure above the disk's center to facilitate parking, e.g. a load/unload ramp structure.
An implementation of a single-sided disk drive wherein the platter is mounted directly to a flat motor offers the additional advantages of reduced motor height and increased motor efficiency. The motor may have a diameter limited only by the dimensions of the disk itself, providing the greatest torque at the smallest motor height.